Band-pass selector



Sept. 9, 1941,. H. A. WHEELER BAND-PASS SELECTOR Filed Oct. 22, 1955 @c EEES E.. :QE NQEEE C5305: o

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VENTOR.

ATTORNEY.

Patented Sept. 9, 1941 amp-Pass sll'mc'ron.4

Harold A. Wheeler, Great Neck, N. Y., assignor to Haaeltine Corporation, a corporation of Dela- I application october zz, 1935, serial No. 4s,os1

9 Claims.

This invention relates to modulated carrier signal receivers, and particularlyv to signal-selecting systems of such receivers and methods of,

land means for, controlling the selectivity and fidelity thereof to discriminate against undesired signals. i l

vIn radio broadcast systems, a signal isordinarily transmitted on a carrier frequency, providing two side bands of signal modulation which usually extend 6 kiiocycles or more on either side of the carrier frequency. In accordance with present practice, broadcasting stations are allotted carrier frequencies uniformly spaced throughout the broadcast frequency range. The spacing of adjacent carrier frequencies is usually 10 kilocycles and, in many instances, the sideband frequencies of one carrier overlap those of adjacent carriers received at the same location, or closely encroach thereon. By reason of this condition it is frequently dlilicult to tune a receiver to a desired signal without interference from undesired signals on carrier frequencies near the desired signal carrier, particularly when the strength of such undesired signals is comparable to or exceeds that of the desired signal. In addition to these undesired signals, static and other so-called background noises, which are ordinarily present at the frequencies of the side bands, may interfere with quiet operation.

In order to obtain faithful reception of theA Y broadcasting correspond to the higher audio frequencies of modulation, are suppressed. Accordingly, it is desirable that the width of the selected band of frequencies be contracted only when an undesired signal of sufficient amplitude to cause interference, or noise, is present, and that in its absence the width of the selected band should be expanded sufficiently to admit and pass lall of the side-band frequencies of the desired signal.

Various arrangements have heretofore been devised for controlling the expansion and contraction of the band of frequencies passed by a selecting system. In certain of these arrangements, the expansion and contraction of the band has 'been automatically controlled in accordance with the amplitude of the desired signal, so that when the desired signal is weak the band passed will be contracted.` 'I'his manner of control, however, is not wholly satisfactory since, as has been pointed out, the amount of contraction required for optimum reception depends not only on the desired signal intensity but also, and infact primarily, on the requirements for avoiding interference on the undesired signal frequencies near the desired signal carrier frequency. Even though the desired signal is strong, the undesired signals may be sufficiently strong to require contraction of the r`band. 'I'he undesired signals most likely to cause interference are, naturally, those having carrier frequencies which are separated'by only l0 kilocycles from the carrier frequency of the desired signal.

In addition to properly controlling the selectivity of a receiver, in order to obtain faithful reception, it is also necessary to control the amplication in the receiver 'so that the output thereof will be maintained within a relatively narrowrange of amplitude for a wide range of input signal amplitudes. Various well-known arrangements, which are commonly referred to as automatic `amplification control, automatic volume control, or A. V. C., have been satisfactorily employed for this purpose. Ideal operation of a modulated carrier receiver, therefore, requires automatic expansion and contraction of the selected band of frequencies according to the amplitude of the undesired signals as well as according to theamplitude of the desired signal, together with the control of the amplification of the signals so as to maintain the output of the receiver within a relatively narrow range of amplitude for a wide range of received signal amplitudes.

Itis a primary object of the present invention, therefore, to provide a method of, and means for, automatically controlling the selectivity of a modulated carrier signal receiver to obtain maximum fidelity of reception consistent with the intensity of undesired signals near the desired signal carrier frequency.

It is a further object of the invention to provide a method of, and means for,'automati`cally controlling the selectivity of a modulated carrier signal receiver to obtain maximum fidelity of reception consistent with the relative'intensities ofthe desired signal and undesired signalsl on frequencies near the desired signal carrier frequency.

- It is a still further object of the invention to provide, in a modulated carrier signal receiver, a method of, and means for, automatically controuing the ampiifbancn of signals therein. to

maintain the amplitude of the receiver output within a relatively narrow range for a wide range of input signal amplitudes, and for simultaneously automatically controlling the selectivity of the receiver to obtain optimum fidelity permitted by the relative intensities of the desired signal and undesired signals, as above mentioned.

lFor a better understanding of' my invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In accordance with a feature of the'invention, there is included in a signal-receiving system the combination of means providing two tuned coupled signal circuits, an electronic control device providing a variable impedance across at least one of the circuits, and means for decreasing the impedance of the control device in accordance with increases in the average amplitude of received signals of one frequency band and for increasing the impedance of said control device in accordance with increases in the average'amplitude of received signals of frequencies adjacent to the first-mentioned frequency band.

Also in accordance with a feature of the invention, there is provided in a modulated-carrier signal received a band-pass selector for selecting a desired signal comprising a carrier frequency and a band of modulation frequencies. The arrangement includes means for adjusting the width of the band of frequencies passed by the selector, means for automatically controlling the adjusting means to expand and contract the band, respectively, with increasing and decreasing'amplitude of the desired signal input to the selector, and means for automatically adjusting the amplitude of the desired signal input inversely in accordance with the amplitude of the input to the receiver of an'undesired signal having a carrier frequency near lthe carrier frequency of the desired signal. By this arrangement. the band is contracted suliiciently substantiallly to limit the amplitude of the undesired signaloutput from the selector. I

Also in accordance with a feature of the invention, there is provided in a modulated-carrier signal receiverV a band-pass selector for selecting a desired signal comprising a carrier frequency and `a band of modulation frequencies. In this arrangement, the selector comprises a primary circuit and a tuned secondary circuit coupled thereto and an auxiliary sharply-tuned trap circuit coupled to the secondary circuit. The primary and secondary circuits are coupled by less than optimum coupling ,and tend to limit the transmission of the selector lto a relativelylnarv signal. Y

In the accompanying drawing, Fig. 1 is a cir- 75 cuit diagram of a complete superheterodyne receiver, partly schematic, employing the present invention; and Figs. 2-5 are graphs representing certain operating characteristics of the receiver to aid in the understanding of the invention. Fig. V2 showing the characteristic of the intermediate-frequency amplifier, Fig. 3 illustrating the characteristics of the adjustable band-pass selector, Fig. 4 representing the resulting extreme 'conditions obtained in the amplifier as a whole, and Fig. 5 illustrating generally the ultimate results obtained by the present invention.

Referring now more particularly to Fig. l, there is here shown schematically a s uperheterodyne radio receiver, employing my invention in a preferred form. In general, the receiver includes a s tunable radio-frequency amplifier and frequency changer 'I having its input connected with an antenna III and a ground II and its output connected to an intermediate-frequency amplifier 8 by means of a band-pass selector system I2 which forms a part of the present invention and is hereinafter described in detail. A selective coupling network I3v serves to couple the output of the intermediate-frequency amplifier 8 to a detector and audio-frequency system 9. The selective coupling network I3 includes an intermediatefrequency transformer I4 having a primary winding I5, which is included in the output of the last stage of intermediate-frequency amplification and is tuned to the intermediate frequency by a condenser I6, and a secondary winding I1, which is included in the input of the detector and audio-frequency system and is tuned to the intermediate frequency by a condenser I8.

The primary circuit of the transformer I4 is damped by parallel resistor I9, and the damping of the primary and secondary circuits is proportioned so that the primary circuit is broadly resonant and the secondary circuit is sharply resonant. The coupling of the transformer is on the order of optimum, preferably somewhat greater than optimum, and the arrangement thus provides a selective quieting action, by reason of the Aselectivity of the secondary circuit, which is entirely subsequent to the automatic control provided by the present invention as hereinafter described.

It will be understood that the tunable radiofrequency amplifier and frequency changer 1, the intermediate-frequency amplifier 8, and the'detector and audio-frequency system 9 may be of conventional construction Yand operation, the details of which are Well known in the art, rendering description thereof unnecessary herein.

Neglecting for the moment the particular operation of the selector systems I2 and I3 and the apparatus associated therewith constituting the principal features of the present invention as described hereinafter, the system described above comprises a conventional modulated-carrier signal receiver of the superheterodyne type for receiving a desired signal comprising a carrier frequency and a band of modulation frequencies. The operation of such a receiver is well understood in the art and a detailed explanation there- 'of is, therefore, unnecessary. In brief, signals audio-frequency system l, wherein the audio-frequency signals are derived, amplified and supplied, in the usual manner, to a loudspeaker for reproduction.

While, in accordance with the present invention, any suitable selectivity control circuit per se may be employed for adjusting the width of the band of frequencies passed by the selector, in the preferred arrangement shown in the drawing, the band-pass selector l2 is provided for this purpose. The selector I2 comprises an intermediate-frequency transformer 2|, having a primary winding 2l included in the output circuit of the radio-frequency amplifier and frequency changer 1 and a secondary winding 22 included in the input circuit of the intermediate-frequency amplifier 8. An adjustable fixed condenser 23 is connected across the secondary circuit as illustrated, serving to tune this circuit to the intermediate frequency. Also associated with the transformer is an auxiliary trap or tertiary circuit 24, comprising a winding 28 and condenser for tuning the Winding 28 to the intermediate frequency. The coupling between'the primary and secondary windings 2l and 22 is considerable but less than optimum and not sufficient to damp the secondary circuit appreciably, so that, independent of the trap circuit, the transmission of a. relatively narrow band of frequencies is effected. The secondary winding 22 and tertiary winding 25, however, have a coecient of coupling considerably greater than optimum, preferably approximately double the ratio of the highest desired frequency of modulai tion to the intermediate frequency, so that the effect of the trap circuit is to expand the pass band of the selector system to a relatively broad band of frequencies.

In order automatically to lcontrol the effectiveness of the trap circuit to expand and contract the pass band thereof, respectively, with increasing and decreasing amplitude of the desired signal input to the selector, and thereby control the selectivity of the system, means are provided for automatically damping the trap circuit. While any suitable arrangement may be employed for this purpose, in the preferred embodiment of the invention a vacuum tube 26 is employed, with its space path connected across the trap circuit, and means are provided, as hereinafter further described, for applying a negative bias voltage to the grid of this tube to control the tube conductance and hence its damping effect on the trap circuit. A suitable voltage source, such as a battery 21, is provided for supplying the space current of the tube 26.v The anode and grid of the tube are connected through a coupling condenser 28, and a leak resistance 29 is included in the grid circuit. The condenser 28 and resistor 29 serve to provide a degenerative effect which greatly increases the damping to which the trap circuit can be subjected by the tube. The tube 26, as well as the tubes of the intermediatefrequency amplifier 8, are preferably of the variable-mu type having a gradual cutoff, in order that distortion of the signal envelope may be avoided.

In order to produce the control bias voltage for the tube 28 and for controlling the amplification in the tubes of the intermediate-frequency ampliiier, a diode rectifier 3| is coupled to the output circuit of the intermediate-frequency amplifier through coupling condenser 32. An intermediate-frequency choke coil 33 is connected between the cathode and ground. The load circuit of the rectifier 3l comprises a resistor 38 and a by-pass condenser 34, the bias voltage being built up across this resistor in the usual manner. In order to insure that the biasing voltage be proportional to the intermediate-frequency carrierl and unresponsive to the modulation voltages, there is provided, in addition to the condenser 34 and resistor 35. a filter comprising a series resistor 38 and shunt condenser 31 proportioned to have a time constant which it greater than the period of the lowest audio-frequency components of the signal modulation. 'I'he bias voltage thus developed is applied in a negative sense to the control grids of the amplifier tubes in the intermediate-frequency amplifier, in the usual manl ner| to decrease the amplification therein with increasing carrier amplitude of the intermediatefrequency signal input to said amplifier, thereby to maintain the output of the amplifier within a relatively narrow range for a wide range' of input amplitudes of the desired signal carrier. 'Ihe bias voltage across resistor 35 is also applied through a resistor 28 to the control grid of the tube 26 to control the damping of the circuit 24. VThe effect of the circuit 24 in the selector i2, and hence the selectivity of the receiver, is thus controlled. In other'words, expansion and contraction of the width of the band passed by the selector I2 are effected automatically with increasing and decreasing amplitudes, respectively, of the desired signal carrier input to the selective intermediate-frequency amplifier 8.

The amplitude of the desired signal carrier input tothe intermediate-frequency amplifier is thus the factor which directly determines the degree of amplification therein and the band width of the selected band. In the absence of undesired signals, this general relation i s sufficient for satisfactory reproduction and vno other type of control is needed. The arrangements of the prior art, as hereinbefore mentioned, have been based on this relation. In accordance with the present invention, however, means are provided for automatically adjusting the amplitude of the desired signal input inversely in accordance with the amplitude of the input to the receiver of the undesired signal carrier, specifically, for reducing the amplitude of the desired signal carrier input to the intermediate-frequency amplier with increasing amplitude of an undesired signal on a frequency near the desired signal carrier frequency. Hence, the presence of an undesired signal capable of causing interference will indirectly effect the contraction ofthe band of frequencies passed by the selector in the same manner as a reductionl of the desired signal intenslty andthe band width is contracted sufficiently substantially to limit the amplitude of the undesired signal output from the selector.

This control by undesired signals is accomplished, vin the preferred form of the invention shown in the drawing, by the provision of an auxiliary intermediate-frequency amplifier 38 connected to the output of the frequency changer and having its output, in turn, connected to an automatic volume control or first A. V. C. rectifier 38. 'I'he radio-frequency amplifier 'l is designed to pass a band of frequencies which is at least as wide as the fully expanded band of the intermediate-frequency amplifier 8, this relation being essential to obtain the full benefit of expansion. 'I'he intermediate-frequency ampliiler 38, however, is designed to pass a band of frequencies which is substantially wider than that of the radio-frequency amplier; that is, it

l passes andamplies not only the desired signal I but also all undesired signals which are passed l by they radio-frequency' ampliner and which l have sufficient amplitude to be capable of over- 1 loading the frequency changer or causing inl terference. VThe rectifier 3l is designed and op- 1 erates in the conventional manner to develop a 1 bias voltage -proportional to the amplitude of the signal, supplied thereto. The bias voltage l thus-developed is applied negatively to the control grids of one or more of the tubes in the 3 radio frequency amplifier' and frequency 1 changer 1 to decrease the amplification therein j proportionally to the input of the rectifier 3l. i Hence, in the presence of interfering signals, the desired carrier input amplitude to the intermediate-frequency amplifier l is reduced, with the l resultant contraction of the band 'as previously explained.

The rectifier 3.9, together with the rectifier 3|,

i thus provide a double automatic volume con- 1 trol,` the control by the rectifier 39 serving to l keep the signal level well below the overload level in the first part of the receiver, and the control provided by the rectifier 3| serving. as above described, to maintain the amplitude of g the input to the detector 9 substantially unl- 1 form'. By thus avoiding overloading of the re.

ceiver, distortion of the desired signal and crossmodulation of the desired signal by undesired f ence are those separated from the desired signal ,in the over-all response of the intermediate-frecarrier by about 10 kilocycles or, in other words, l

l are signals on carrier frequencies adjacent the carrier frequency of the desired signal, the inl termediate-frequency amplifier 38 in the preferred embodiment of the invention is designed to have a response characteristic such as that illustrated by the curve of Fig. 2, that is, this g amplifier 'is designed to be'more responsive on adjacent carrier frequencies than on the dei.sired carrier frequency and other frequencies.

In Fig. 2, as also in Figs. 3 and 4, relative gain l in decibels is plotted against frequency differ- 1 ence from the intermediate carrier frequency, in

kilocycles.

resented in Fig. 2, the adjacent undesired signals have relatively a substantially increased ef- Referring now to the general operation of the l receiver, the curves of Fig. 3 illustrate the effect of the. trap circuit on the amplification and band width of the intermediate-frequency selector with which the trap is associated. YCurve 38 illustrates the condition when a minimum bias voltage is being applied to the tube 26 as the result of'a minimum carrier input amplitude to the amplifier B. Under these conditions there is i a maximum damping'of the trap circuit 2l and l consequently'the band width is fully contracted. Curve illustrates the amplification characteristic corresponding to the opposite conditions, that is, a maximum bias on the tube 26, resulting from a maximum carrier input amplitude By virtue of the characteristic Vreponly by the action of the bias voltage applied directly to one or more of the amplifier tubes, but also by the action of the trap circuit 2l. Thus,

the negative bias which is applied to the tube 26 reduces the intermediate-frequency amplification by an amount comparable to the reduction obtained by the control of an amplifier tube. These effects are preferably proportioned to maintain substantially uniform amplification in this amplier at frequencies 10 kilocycles above and below the intermediate carrier frequency.

Fig. 4 shows the resulting extreme conditions quency amplifier and preceding apparatus. The curve 40 represents the condition with a minimum input to the intermediate-'frequency amplifler and hence a minimum application of bias voltage to the tube 28 with consequent contraction of the band width, while the curve Il representsthe opposite conditions. Anlincrease in the negative bias voltage developed by the rectifier Il thus -has the eifect of flattening the peakof the characteristic curve of the amplifier and proportionally increasing the band width. Hence, it may be said thatthe amplification of they desired signal in the amplifier is automatically reduced with increasing carrier amplitude of the intermediate-frequency` signal input to the amplifier, while the amplification at the two adjacent carrier frequencies 10 kilocycles from the desired signal c arrier` frequency is maintained substantially unchanged'. The amplitude of the input to the amplifier of signals on the adjacent carrier frequencies is automatically maintained within a predetermined limit by the amplification control rectier and the band is contracted suiliciently to maintain the amplitudes of undesired signal output from the amplifier less than a predetermined small value.

Fig. 5 is a graphical representation of the ultimate results to be obtained by the present invention, the relative intermediate-frequency band width, in kilocycles, being plotted against the relative received signal intensities, in decibels, for'three different conditions of reception. Curve 42 corresponds to the condition where no undesired signals capable of causing interference are being received. When the signal intensity is small, the band width is contracted, expansion taking place to a predetermined extent as the signal intensity increases suiliciently above the background noise level. In the presence of strong undesired signals, a much greater signal intensity is required to cause the same amount of expansion. Thus, curve 43 illustrates the operation of the system when an undesired signal of sufllcient amplitude to cause interference is being received, while curve vIl illustrates the operation when an undesired signal of still greater amplitude is being received. '.l'lriis constitutes the ideal operation of automatic selectivity control; that is to say, the control circuit performs automatically the same operations as would be performed manually by a skilled operator havingall the various properties of the receiver under his control.

The intermediate-frequency amplifier may advantageously be so designed that'the ratio of the peaks to the valleys in Fig. 2 is about v20 decibels. Increasing this ratio causes further reduction of expansion of the band width in the presence of undesired signals, with corresponding further reduction of interference. A representative range of expansion is one and onehalf octaves, corresponding to band widths from to 14 kilocycles. In terms of audio-frequency output of the receiver, this corresponds to upper cut-0H frequencies of from 2500 to 7000 kilocycles.

In summary, `therefore, it is seen that by the arrangement of Fig. 1 there is included, in a signal-receiving system, the combination of means providing two tuned coupled signal rcircuits 25, 30 and 22, 23 and an electronic control device 26 providing a variable impedance across at least one of the circuits, specifically, across tuned circuit 25, 30. The arrangement comprises means including rectiiler 3|- for increasing the impedance of the4 control ldevice 26 in accordance with increases in the average ampli` tude of received signals of one frequency band, specifically, in' accordance with the desired frequency band, and for increasing the impedance plitude of the desired signal input to said selector, and means for automatically adjusting the amplitude of said desired signal input inversely in accordancewith the amplitude of the input to said receiver of an undesired signal having a carrier frequency near the carrierfrequency of said desired signal, whereby said -band is contracted sumciently to substantially limit the amplitude of the undesired signal output from said selector.

2. In a modulated4 carrier signal receiver, a band-pass signal amplifier for amplifying a desired signal comprising a carrier frequency and a band of modulation frequencies. means for varying the signalamplification in said amplier inversely in accordancel with the amplitude of the desired signal carrier input to said amplifier to maintain the output thereof within a relatively of the control device in accordance with increases of the average amplitude of received signals of frequencies adjacent to the desired frequency band.

While the invention has been described as employed in connection with a receiver designed for the reception and reproduction of a carrier and both side bands of 'modulation frequencies, it will be readily appreciated that the essential features of the invention are equally applicable to receivers designed for signal side-band reception, for example, such as the receivers described in Patents 2,050,679 and 2,050,680 granted to applicant on August 11, 1936. The features of this invention are also particularly well adapted for use in connection with the receiving systems disclosed in Patent 2,054,412 granted to John F. Farrington on September 15, 1936; Patent 2,038,358 'granted to Daniel E. Harriett onv April 21, 1936; vand Patent 2,073,344 granted to John Kelly Johnson on March 9, 1937. For such a receiver, the circuits embodying the present invention will be designed accordingly for properly adjusting the width of the single side band which is passed. The invention thus contemplates means for adjusting at least the width of the part of the selected band on one side of the desired carrier frequency inversely in accordance with the input amplitude of an undesired signal on a frequency near said band and on the same side of the carrier frequency.

' While I have described what I at present consider the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from my int vention, and I, therefore, aim in the appended.

narrow range for a wide range of input signal amplitude and for simultaneously adjusting the width of the band of frequencies passed thereby, directly in accordance with the amplitude of the desired signal carrier input to said amplifier, and means for automatically adjusting the amplitude of said carrier input inversely in accordance with the amplitude of the input to said receiver of an undesired signal on a frequency near saidband, said means being mutually proportioned to contract the Width of said band automatically suiiiciently to maintain the amplitude of said undesired signal less than a predetermined small value in the output of said amplifier.

3. In a modulated carrier signal receiver, a band-pass signal amplifier for selecting a desired signal comprising a carrier frequency and a band of modulation frequencies, means for varying the signalA amplification in said amplifier inversely in accordance with the amplitude of the desired signal carrier input to said amplifier to maintain the output thereof within a relatively narrow range for a Wide range of input signal amplitude and for simultaneously adjusting the width of the band of frequencies passed thereby directly in accordance with the! amplitude of thedesired signal carrier input to said amplifier, and means substantially more responsive at frequencies near the desired signal carrierfrequency than at the desired signal carrier frequency and other frequencies, for automatically adjusting the amplitude of said carrier input inversely in accordance with the amplitude of ai'undesired signal on a frequency near said band, said means being mutually proportioned to contract the width of said band automatically sufiiciently to maintain the amplitude of the undesired signal less than a predetermined'small value in the output of said amplifier.

4. In a modulated carrier signal receiver, a band-pass selector for selecting a desired signal comprising a carrier frequency and a band of modulation frequencies, said selector comprising a primary circuit and a tuned secondary circuit coupled thereto, and an auxiliary sharply tuned trap circuit coupled to said secondary circuit; said primary and secondary circuits being coupled by less than optimum coupling and tending to limit the transmission of the selector to a relatively narrow band of frequencies, and said secondary and trap circuits being coupled by considerably more than optimum coupling to expand the band passed by the selector, means for damping the trap circuit to reduce. its effect on said selector, and means responsive to the amplitude of an undesired signal on a frequency near the desired signal carrier frequency for automatij cally controlling said damping means to adjust l the widthof the band of frequencies passed said selector inversely in accordance with the l amplitude of said undesired signal.-

5. In a modulatedcarrier signal receiver, a

band-pass selector for selectingva desired signal comprising .acarrier frequency and a band of l modulation frequencies, saidy selector comprising a primary circuit and a tuned secondary circuit l coupled thereto, and an auxiliary sharply tuned 1 trap circuit coupled to said secondary circuit;

said primary and secondary circuits being coupled by less than optimum coupling and tending to i limit the transmissionfof the selector to a rela- Q tively narrow band of frequencies, and said sec- 1 ondaryand -trap circuits being coupled by considerablymore than optimum coupling to expand the band passed by the selector. a vacuum 1 tube conductance connected across said trap circuit for damping said trap circuit to reduce its eilect on said selector, means for developing a bias voltage having an amplitude varying directl ly with the Vinput carrier amplitude of the desiredsignal, means for negatively applying said I voltagetoa control electrode'of said vacuum tube conductance whereby the width of the band g of frequencies passed by said selector is adjusted,

j and means for modifying said input carrier amplitudinversely in accordance with the amplitude loi' an undesired signal on a frequency near the desired signal `carrier frequency.

6. A radio receiving system having a plurality oi' cascade-coupled ampliilers and including l means for altering the selectivity thereof in one 7. A radio receiving system having a plurality v 3 of cascade-coupled amplifiers and" including lfirst mentioned frequency band.I

'assumo means for altering the selectivity thereof in one sense in response to variations in the amplitude of a potential derived from electrical energy in a limited range of frequencies and in the opposite sense inresponse to variations in the amplitude of a potentialderived from electrical energy in frequency bands adjacent to said limited range, inrwhich said means comprise at least one coupling device for at least two of said amplifiers. said coupling device including' two tuned circuits intercoupled, an electronic control device providing variable impedance means connected in parallel with at least one of said tuned circuits. and means for applying said control potentials to said variable impedance means to cause the impedance thereof to vary and thereby to alter the selectivity of said'coupling device in opposite sense.

8. In a radio receiving system, the combination of a plurality of electronic amplier devices, coupling means between at least two ofsaid, devices comprising a pair of coupled resonant circuits, an electronic control device connected in f parallel with one of said resonant circuits," 'and, xneans for impressing control potentials on said cntrolde- .of said control device in accordance with increases in the average amplitude of received signais of one frequency band, and for increasing the impedance of said control device in accordance with increases in the average amplitude of received signals of frequencies adjacent to said HAROLD A. WHEELER. 

